Having recently succeeded in preparing in three species antibodies to the purified estrogen receptor protein (estrophilin) of calf uterus and having shown that these cross-react with estrophilin of reproductive tissues of all mammalian species tested, we are attempting to prepare substantial quantities of the purified receptor, leading, in turn, to sufficient amounts of the antibody to permit a variety of immunochemical studies. These include the purification of the immunoglobulin to yield an antibody suitable for labeling to provide a simple radioimmunoassay for estrophilin in mammalian tissues and tumors. We propose to use the antibody linked to a supporting medium as an efficient means for receptor purification and to employ peroxidase of ferritin-labeled antibody for the precise intracellular localization of estrophilin by electron microscopy. Studies are being continued of the silver ion-induced release of receptor bound estradiol and its reversal by dithiothreitol as a convenient means of determining estrophilin that is bound to endogenous hormone in target tissues.